This application claims the priority of German patent document 197 12 087.3, filed Mar. 22, 1997, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a combination of an adsorbent with a catalyst for internal combustion engines.
Catalysts, as well as adsorbents, are being used increasingly for detoxifying exhaust gases from internal combustion engines, in order to avoid emissions after the cold start of the internal combustion engine, that is, when the catalyst has not yet reached its operating temperature. These adsorbents or catalysts are ceramic or metal supports with a honeycomb structure, which have been coated appropriately. After the cold start of the internal combustion engine, the adsorbent is intended to adsorb certain components of the exhaust gas here, especially hydrocarbons or NO.sub.x, and to store them until the downstream catalyst has reached its operating temperature. In this connection, the problem arises that the desorption temperature of the adsorbent, that is, the temperature above which the adsorbent emits the adsorbed components of the exhaust gas once again, must be higher or at least equal to the temperature at which the conversion of said exhaust gas components commences in the catalyst (light-off temperature).
Moreover, it has become known, for example, from B. R. Powell, S. E. Whittington: "Encapsulation: A new mechanism of catalyst deactivation", Physical Department, GM Research Lab., Warren, Mich., U.S.A., in: J. Catal. 1983, that a coating of zeolites (adsorbing action), coated with noble gas (catalytic action), used in the exhaust gas stream, loses its effectiveness at high temperatures between 930.degree. C. and 1000.degree. C., because the noble metal particles sink into the silica surface present in the zeolite. A silica bead is formed here around the base of the noble metal particle, encapsulating it, so that the effective surface area of the noble metal particle is drastically reduced. It follows from this that a coating, which is formed jointly from zeolites and noble metal particles, is not capable of functioning permanently.
Coatings for adsorbents and catalysts have become known, for which the desorption temperature of the adsorbing coating lies within the range of the light-off temperature of the catalytically active coating.
It is an object of the invention to provide an adsorbent-catalyst combination, which is optimized with respect to its storage and conversion behavior.
This and other objects have been achieved by providing an adsorbent-catalyst combination for an internal combustion engine, comprising at least one adsorbing zone of an adsorbing material and at least one catalytic zone of a catalytic material, said adsorbing zones being arranged alternately with said catalytic zones in an exhaust gas stream.
This and other objects have also been achieved by providing an adsorbent-catalyst combination for an internal combustion engine, comprising: a housing defining a flow path to be communicated with an exhaust stream of the internal combustion engine; at least one adsorber arranged in said housing; and at least one catalyst arranged in said housing, said at least one adsorber and said at one catalyst being arranged alternately along said flow path.
This and other objects have also been achieved by providing a method of manufacturing an adsorbent-catalyst combination for an internal combustion engine, comprising: providing a housing defining a flow path to be communicated with an exhaust stream of the internal combustion engine; providing at least one adsorber; providing at least one catalyst; and arranging said at least one adsorber and said at one catalyst alternately along said flow path of the housing.
According to the invention, it is proposed for this purpose that the adsorbent-catalyst combination is built up in alternating layers of adsorbing and catalytically coated zones, which are connected in series with respect to the exhaust gas stream. By separating the adsorbing and catalyzing zones, a mutual effect of the materials used here is precluded; an integral coating of adsorbing material and of material with catalyzing activity is reproduced roughly in that a division is made into regions, which are coated only with adsorbing materials and regions, which are coated only with catalyzing materials. In other words, steps of an adsorbing region and a downstream region of catalytic activity are linked together.
It is particularly advantageous if the downstream regions of catalytic activity become active before or insignificantly after the desorption of the upstream adsorbing region commences. Without further measures, however, the upstream, adsorbing region acts as a heat sink and therefore heats up more rapidly than does the subsequent downstream region of catalytic activity. Moreover, the subsequent downstream region of catalytic activity attains its effectiveness (light-off temperature) clearly later than in a system without the proposed upstream, adsorbing region.
Since the desorption temperature of the upstream, adsorbing region lies in the region of the light-off temperature of the subsequent downstream region of catalytic activity, the desorption commences although the subsequent region of catalytic activity is not yet effective. The desorbed materials are therefore not converted in the region of catalytic activity. In addition, as described above, since the region of catalytic activity attains its effectiveness later than it would in a system without a preceding adsorbing region, the total emission is higher than it would be in a strictly catalytic system without a preceding adsorbing region.
It is therefore proposed that the system of adsorbing regions and of regions of catalytic activity be formed in such a manner geometrically and from a material point of view that;
a) there is a large temperature difference between the exhaust gas and a support material in the region of adsorbing activity; PA1 b) a small temperature difference between the exhaust gas and the support material in the region of catalytic activity; and PA1 c) a small temperature difference between the inlet temperature and the outlet temperature in the region of adsorbing activity.
This achieves, at least in the inlet region of the region of catalytic activity or in the whole region of catalytic activity, that the temperature of the support material is higher than the temperature in the outlet region of the upstream adsorbing region or in the whole of the upstream adsorbing region. Accordingly, the region of catalytic activity can reach its light-off temperature before the start of the desorption in the adsorbing region, even if the desorption temperature is somewhat lower than the light-off temperature.
With this design, the effectiveness of the region of catalytic activity sets in before the desorption process in the adsorbing region commences. Passage of hydrocarbons through such an adsorbent-catalyst combination can therefore be reduced significantly or prevented completely.
With the proposed formation of the adsorbent-catalyst combination, effective purification of the exhaust gas after a cold start of the internal combustion engine is possible, without the need for additional heating systems, such as electrical heating or a burner, for this purpose.
Beyond the use in the present adsorbent-catalyst combination, the proposed formation can also be used advantageously when a catalyst is connected downstream from a different system. This is particularly the case in systems, with an adsorbent coating having catalytic activity (integral coating) and a downstream catalyst, as well as in systems with a known arrangement of a support, coated only with adsorbing material and having a downstream catalyst.
In order to attain this objective, it is proposed that zones with adsorbing activity be designed thermodynamically in such a manner that, compared to the zone with catalytic activity, they have a lower heat transfer or a higher heat capacity or both. This can be accomplished, for example by one or several of the following measures for the design of the geometry and material of the adsorbing zones: a larger wall thickness, a lower cell density, a higher heat capacity, a higher specific heat capacity, a higher density of the support material, a lower surface area or no decreased surface or a decreased surface structure (smooth surface). On the other hand, for the design of the geometry and material of the zones having catalytic activity, the following opposite measures can be employed: a lesser wall thickness, a higher cell density, a lower heat capacity, a lower specific heat capacity, a lower density of the support material, a larger surface area or a larger surface structure (rough surface, flow interruptions, flow faults, flow deflections). The advantage, achieved herewith, lies in the smaller difference between the temperature of the exhaust gas and that of the zones of catalytic activity than between the temperature of the exhaust gas and that of the zones of adsorbent activity. This smaller difference makes it possible to have higher temperatures in the inlet region of a zone, which has catalytic activity and is downstream from an adsorbing zone, than in the outlet region of the adsorbing zone.
In additional, fundamental considerations of the present adsorbent-catalyst combination, it was recognized that a rapid rise in the temperature of the adsorbent-catalyst combination brings about a rapid attainment of the light-off temperature of the region of catalytic activity and, with that, a short time, in which the hydrocarbons must be adsorbed. A shortening of the heating time of the region having catalytic activity up to the light-off temperature thus enables the volume of the adsorbing region to be reduced.
A measure of the heating rate, that is, of the amount of heat that can be adsorbed in unit time, is given by the product of the thermal conductivity, the density and the heat capacity of the support used. The higher this product, the more heat can be adsorbed by a given support in the same time for the same increase in temperature. Knowing this relationship, the rate of heating of the adsorbing and catalytic regions can be adjusted optimally for a given mass flow and a given temperature of the exhaust gas. As explained above, this can be done by selecting the material properties of the support in the adsorbing and catalyzing regions.
In order to achieve a short heating phase and, with that, a small volume for the adsorbing region, the density and the specific heat capacity of the support for the adsorbing region as well as of the support for the region of catalytic activity should be low. The volume, selected for the adsorbing region, must be sufficiently large so that the amount of hydrocarbons, arising from the exhaust gas stream supplied by the internal combustion engine, can be adsorbed before the light-off temperature of the downstream catalyst is attained. The volume of the region of catalytic activity must be sufficiently large so that complete conversion of the materials, contained in the exhaust gas stream, is possible at any operating point of the internal combustion engine.
In addition, the temperature distribution in the support material, which is affected by the thermal conductivity of the support material, can also be taken into consideration. A high thermal conductivity favors a more uniform distribution of temperatures in the axial direction more so than does a low thermal conductivity. Accordingly, a high thermal conductivity is advantageous for the adsorbing region, since temperature peaks can be dissipated by it and, with that, the start of the desorption can be delayed.
The available surfaces and the masses to be heated are to be considered as further parameters affecting the heating time. Supports with a smaller mass and a large surface area shorten the heating time, while supports with a larger mass and a small surface area prolong the heating time.
Finally, aside from the temperature, it is also possible to take the concentration of the material to be adsorbed (hydrocarbons here) from the exhaust gas into consideration for the absorption. A low support temperature in the adsorbing region and high hydrocarbon concentrations are advantageous for a high adsorption of hydrocarbons. On the other hand, a high support temperature in the adsorbing region and a low hydrocarbon concentration are advantageous for the desorption.
It is furthermore proposed that the depth of the individual zones be selected so that a matrix temperature at the inlet of a zone of catalytic activity already lies above the light-off temperature when the matrix temperature at the outlet of the upstream adsorbing zone is still below the desorption temperature.
It is also advantageous to provide a zone of catalytic activity as the last zone. By this measure, it is ensured that the components of the exhaust gas, emerging from the last adsorbing zone (as seen in the flow direction), are also reacted and cannot emerge from the adsorbent-catalyst combination.
Furthermore, it is proposed that the adsorbent-catalyst combination be built up as a stack of disks of support material, which have the same external diameter, an optionally different length and are coated alternatively so as to have an adsorbing action or a catalytic action. Known supports, such as metallic or ceramic supports or combinations thereof, can be used for this construction. The disks can follow one another with or without an interval. Moreover, with this construction, it is possible to standardize the individual components of the combination and, by the number of supporting disks used, adapt them to the volume of catalytic or adsorbing material required for the purification of the exhaust gases. The manufacturing costs can be clearly lowered by this standardization.
The invention can be used not only for the combination of adsorbing zones and zones of catalytic activity, but explicitly also for the combination of zones, which are both adsorbing and have catalytic activity with downstream zones having catalytic activity.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.